CI: run release drafter on legacy branches (prebid#14124)

* CI: run release drafter on legacy branches

* Remove unnecessary blank line in workflow file

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Co-authored-by: Patrick McCann <patmmccann@gmail.com>

Author: dgirardi

.github/workflows/release-drafter.yml

@@ -5,6 +5,7 @@ on:
     # branches to consider in the event; optional, defaults to all
     branches:
       - master
+      - '*.x-legacy'
 
 permissions:
   contents: read